Recent efforts for fabrication of photovoltaic devices have included developing cost-effective thin film solar cells with reasonable efficiencies as alternatives to traditional silicon-based solar cells. The core structure of such thin film solar cells typically contains a photovoltaic absorber layer of a chalcopyrite or a kesterite compound. An absorber layer made of a chalcopyrite compound typically contains elements from each of groups IB, IIIA, and VIA of the periodic table of elements, including copper and indium and/or gallium or aluminum and selenium and/or sulfur, denoted by Cu, In, Ga, Al, Se and S in commonly accepted chemical symbols. For such materials to perform as p-type solar absorbers and to have the desired carrier type and concentration, the atomic ratio between these group IB:IIIA:VIA elements, e.g., Cu:(In+Ga):Se is strictly controlled near 25%:25%:50% with allowable deviations towards Cu-deficient and Se-rich by percents of plus or minus 15%. This predetermined ratio is known to those of ordinary skill in the art and is adhered to in the design of these solar absorbers. Such absorber layers are often referred to as CIGS layers, and the corresponding devices are referred to as CIGS solar cells. Alternatively, an absorber layer made of a kesterite compound typically contains elements from each of groups IB, IIB, IVA, and VIA of the periodic table of elements, e.g. copper and zinc and tin and selenium and/or sulfur, denoted by Cu2ZnSn(S,Se)4. Such absorber layers are often referred to as CZTS layers, and the corresponding devices are referred to as CZTS solar cells. The atomic ratio between these group IB:(IIB+IVA):VIA elements, e.g. Cu:(Zn+Sn):(S+Se) is also strictly controlled near the predetermined ratio of 25%:25%:50%, with allowable deviations also towards slightly Cu-deficient and (S+Se)-rich by a few percents of plus or minus 15%.
Conventional vacuum-based techniques for depositing the absorber layer include evaporation, sputtering, chemical vapor deposition, and the like. The vacuum-based techniques provide for a well-controlled film composition and composition gradient and they yield absorber layers with high conversion efficiencies. These techniques, however, are of limited use because they require significant capital investment, involve complicated deposition procedures, and result in excessive waste of raw materials. In addition, vacuum-based techniques are not well matched to the large-scale and high-yield industrial manufacturing demand in the light of increasing solar cell panel size.
Generally, non-vacuum techniques are alternative techniques for depositing the absorber layer. These include electrodeposition, spray pyrolysis, paste coating, drop casting, screen printing, and the like. These techniques generally include a wet-process comprising providing a solution, suspension, ink, paste, or paint, which contains absorber precursor materials and may be rapidly coated into a thin layer on a substrate. The precursor layer is then heat-treated to form a polycrystalline absorber film by annealing and/or selenization or sulfurization in a reactive gas of ambient or evaporated selenium or sulfur vapor. The non-vacuum-based techniques are more favorable than the vacuum-based techniques because they provide high efficiencies in raw material utilization, fast and simple deposition procedures, low capital investment and processing costs, and possibilities for manufacturing scale-up. In addition, flexible substrates may be incorporated into these techniques.
One of the most important parameters in fabrication of high quality solar absorber layers is precise control of the chemical composition of the absorber layer, which is critical for providing the absorber layer with a desired crystal structure, a desired electrical conductance, and strong optical absorption in the solar spectrum. In this regard, the non-vacuum techniques have many drawbacks which include: poor reaction controllability; low crystal structure quality; low packing density of the absorber layer; adhesion loss at the absorber layer/substrate interface; and unavoidable impurity contaminations from many reaction additives such as residual reactants, by-products, surfactants, dispersants, and binders etc. which are required in a given process.
To solve these problems, one choice of non-vacuum methods is to synthesize nanoparticles in solution with a chemical composition that is appropriate for solar absorption by carrying out a wet reaction process followed by precipitation of the nanoparticles. U.S. Pat. No. 7,663,057 and U.S. Pat. No. 7,306,823 teach several methods of preparation of nanoparticles in solution that can contain the desired elements with an appropriate atomic ratio between the elements. However, just to synthesize these particles can require well over 48 hours of reaction time as shown in several examples in U.S. Pat. No. 7,663,057. These nanoparticles are then mixed with various additives to form a viscous paste or slurry or ink that can be coated onto a substrate to form a thin film. In these methods, although the nanoparticles have the desired composition, the chemical synthesis of these nanoparticles and the subsequent making of a paste or an ink both require large amounts of additives such as salt precursors, surfactants, binders, emulsifiers, thickening agents, and anti-foaming agents. The synthesis requires additional processing steps such as high temperature heating to remove these extraneous reagents and to form the final absorber layer. In addition, the above mentioned binders, thickening agents, and anti-foaming agents are mostly polymers that contain long chains of carbohydrate. Complete removal of these materials often requires annealing of the absorber layer under an oxygen-rich environment. This annealing process increases the risk of oxidizing the absorber layers, which are mostly sulfides, selenides, and tellerides. The oxidation damages the absorber layer and reduces the device's efficiency.
One method of forming a thin film, electrophoretic deposition (EPD), is a broadly acknowledged non-vacuum coating method employed in automotive, appliance, and general organic industries. During the process of EPD, surface-charged particles suspended in a liquid medium will migrate under the influence of an external electric field and be rapidly deposited onto an electrically conductive or semi-conductive surface having the opposite charge. High density films of metals, ceramics, polymers, semiconductors, or carbon have been deposited as described in the prior art such as in “The mechanism of electrophoretic deposition” by Brown and Salt in J. Appl. Chem., 15, 40 (1965), and in U.S. Pat. Nos. 3,879,276; 4,204,933; 4,225,408; and 4,482,447. The above described prior art all require delicate procedures for making the nanoparticle suspension in solution, which involves chemical synthesis such as a metathetical reaction or a reduction reaction to form the nanoparticles, and the described EPD processes typically required assistance of specific acids or bases, stabilizers, and/or binding agents. In addition, some of the described processes required use of post-deposition high temperature treatments at 300 to 800° C. to form the final film as described in U.S. Pat. Nos. 4,204,933 and 4,225,408. These delicate procedures disclose vulnerability and complexity in EPD process control and increase the processing cost accordingly. In addition, the use of chemical reactants and assisting additives will inevitably result in waste of raw materials and introduce chemical contaminations into the suspension and onto the deposited film. Thus, an EPD process has not found use in the highly desired production of large-scale solar panels.
It is highly desirable to provide a cost-effective, practical, and simple method for forming photovoltaic absorbing layers. Preferably the method requires minimum chemical additives during both formation of the nanoparticles and the subsequent coating of the nanoparticles onto a substrate to form a photovoltaic absorber layer. The method must ensure that the resultant absorber layer contains the desired chemical composition.